# ASTR 160: Frontiers and Controversies in Astrophysics

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# Frontiers and Controversies in Astrophysics

## ASTR 160 - Lecture 3 - Our Solar System and the Pluto Problem

### Chapter 1. Review of the Problem Set [00:00:00]

Professor Charles Bailyn: Okay now, we didn’t have sections this week, in case you didn’t notice, and therefore you didn’t have an opportunity to discuss the problem set during section. So, I thought I would say a few words about it now - here is the problem set. You probably can’t read it at this typeface but that’s okay. So, let’s see, problem zero is just a stupid way of making you read the policies. Never mind that. Problem set one, problem one: here are exercises in one-digit scientific notation. I don’t have rules for this. As I mentioned last time, the only rule is common sense. I think there might be some difficulty with the last one. This business of taking things to the one third power is important because you keep ending up with a cubed equals something, and you have to figure out what to do about that. So, let me not do this particular problem, let me do a different one for you.

Supposing you had (6 x 104). And you might be tempted to say, well okay, that’s 6 times 104/3. And that leads you to a bad place, because 104/3 is not the notation we want. We want this to be an integer up there. What does it mean to be a 1 with 43  of a zero after it? And so, you don’t like that. So, the way to deal with this is to regroup. This is (60 x 103). That’s 60 x 101. What’s 60? Well, 4 times 4 times 4, as it happens, is 64, and that’s close enough for me. So, this is 4 x 101. So, that’s just an example of how these kinds of things where you take things to fractional powers, either the square root, which is to the ½, or the cube root to the ⅓.

All right, the next problem. Let’s see, Neptune’s moon Nereid has an orbital period of almost exactly one Earth year. If the mass of Neptune is something or other, what’s its semi-major axis? So, you have P and M and you’re asked for A. That’s a completely straightforward plug-and-chug problem because there’s an equation that relates these three things and the only tricky thing about this is that you have to make sure the units come out right.

All right, the next one looks similar in form, but it isn’t. Consider a Sun-like star orbited by a planet with a period of eighty years. So, we have P–the separation of the planet and the star appears to be 20 arc seconds. So we have an angle, that’s α. How far away is the star? And you want to know D. Okay, there is no equation that contains all three of those things and so this, although it looks similar in form, is actually a substantially more difficult problem.

Going on, problem four. Okay the important–in fact, there is such a star, blah, blah, does this fact make any difference in the forgoing calculation? Explain. The important thing to note about this is that I’m not asking you to do a calculation. This is not a calculation problem; this is a comment. You’re supposed to say something about how the calculation would go if you were to do it, but you don’t have to actually calculate anything there.

Student: What would you say is too short for the answer to that question?

Professor Charles Bailyn: Oh the answer–well, a sentence is too short, two sentences probably too short unless they’re pretty severe sentences. I mean, it’s got to be a paragraph, you know; otherwise, you haven’t really cleared your throat. But I don’t want to say anything too precise about that. If you’ve got a really keen sentence, you know, that might do the job. Again, sanity ought to prevail. Yes.

Student: Is the answer to a question like that typical of most of the problem sets?

Professor Charles Bailyn: We’ll probably have one of that nature. Not necessarily of this kind about scientific controversies, but about something. Oh, and I should say, the way the tests will work out is it’s going to be probably slightly more than half calculation and slightly less than half other things. There won’t be essays because there probably isn’t time to do that on a fifty-minute test but there’ll be short answer questions and stuff like that. Yeah, we’ll have these kinds of discussion things. They won’t be the dominant part of the problem sets. Just as in this case, it’s six points out of–this adds up to twenty points; this one’s six [points at problem set]. Other questions? Okay, as I say, if you’ve got some, let us know.

### Chapter 2. Conservation of Momentum in Exoplanets [00:06:22]

Okay, so as I pointed out last time, we’re talking about “exoplanets” ‒ planets around other stars. And the problem with exoplanets is you can’t see them directly. So, you can’t see the exoplanets directly as blobs of light in the sky and so what do you do? So, you have to detect them obviously indirectly and here’s–we’ve got to invoke another one of Newton’s laws. So, the key law here is now Newton’s Third Law. Newton’s Second Law is F = ma; you may recall that explains all the Physics 180. Newton’s Third Law is usually phrased in textbooks and stuff as “every action has an equal and opposite reaction.” I hate this formulation because it leads to really bad philosophy.

You know, this is one of the whole problems with physics in general. There are these words that seem to mean something. Words like “force,” words like “potential,” and these have technical meanings in physics that are actually different from the intuitive meaning that you have in your head just from using them in everyday life. The big problem with introductory physics is getting people to use these words as if they mean the physics definition rather than as if they mean the everyday life definition. And this leads, as I say, in extreme cases, to all sorts of bad philosophy. In fact, the two most misused physics words that I know of are, first of all, “relativity”–“Everything is relative,” said Einstein. Well, no, actually, he said nothing of the kind. And the other one is “uncertainty,” which has to do with quantum mechanics and these are technical terms in physics and then they get sort of promoted to use in philosophy as if they meant what they mean in English. So, I don’t like this “every action has an equal and opposite reaction.” You can get into all sorts of bogus philosophy and I don’t want to go there.

So, I would rather phrase this as “conservation of momentum.” And momentum, as I say, is a technical term. It means M, mass, times velocity, that’s all it means. And the kind of use that it’s put to in electoral politics or other things has nothing to do with what’s going on in physics. Oh, I should say: velocity, one should be aware of, consists of two things. It’s not just the speed that something has, but also its direction. And so direction counts. So, if you turn around and go at the same speed you’ve reversed your–you now have negative velocity compared to where you started with. This, technically speaking, makes this a vector quantity, but again, we won’t go there.

Okay, so how does this work in the case of a planetary orbit? So, here’s a planet, it’s on one side of its star, it’s going this way [draws an arrow up] and it’s got some momentum. And then half an orbit later, it’s over here going the other way. Let’s say it’s orbiting some star in the middle and it’s got more or less the same speed but its direction is reversed. So over half an orbit–so over ½ orbit, the momentum is reversed. So, it goes from being positive to negative, or something like that.

Now, Newton’s Third Law says that momentum is conserved. The total momentum of the system has to remain the same; so something else has to change. And here’s the whole trick about finding exoplanets, and that is that the star moves too. So, when the planet’s over here at this blue arrow–the star is here, and it’s also moving in the opposite direction. And then half an orbit later, the planet has come over to this side. The star has moved in the opposite direction; it’s now going this way. And so the star moves too, that’s key.

How much does the star move? Well, the two momenta have to be equal and opposite. This is what is equal and opposite about Newton’s Third Law is that these things have to cancel out. So what you find is that the mass of the planet times the velocity of the planet is equal to the mass of the star times the velocity of the star. And because the mass of the star is so much bigger, so huge compared to the mass of the planet, its velocity must be much, much smaller in order for these two things to be equal.

Okay. What are they orbiting around then, if everything is moving together? They orbit around the “center of mass,” so-called. This is just, you know, where the balance point of a seesaw would kind of be. And so here we have–here’s the center of mass, here’s the star moving this way, way over here somewhere is a planet moving much faster. And let’s define two quantities: this is the distance from the planet to the center of mass. Here’s the distance of the star to the planet of mass. Star is moving at Vstar, planet is moving at Vplanet. And you can define a total velocity, which is equal to these two things added together. And what does that mean? That’s the velocity where if you’re on one of these objects, the other appears to be moving relative to you. So, if I’m going this way and you’re going that way, we have a relative velocity equal to the sum of our individual velocities. And there’s also a total distance, which can be defined, which is, by analogy, the sum of the distance from the planet to the center of mass, and the star to the center of mass. And that’s just the distance between these two objects.

The distance and the velocity in an elliptical orbit can change during the course of the orbit. But for circular orbits or close to circular orbits they remain about the same. Orbits, you’ll remember, are generally elliptical. And a way of saying this is that the maximum of the total distance between these two is this quantity a, the semi-major axis. Remember, the semi-major axis is the long slice through the ellipse. So, when this is at a maximum, that equals the semi-major axis. For orbits that are nearly circular you can sort of say in an offhand way, well this, Dtotal, doesn’t change that much. So, it’s the distance between the two objects.

### Chapter 3. Inner and Outer Solar System Objects [00:14:02]

All right, now as I mentioned, VpMp is equal to VstarMstar. It is also true the way you figure out where the center of mass is, DpMp is equal to DstarMstar. And the whole point of this is that this is a large quantity compared to the mass of the planet, and therefore these are small quantities compared to the distances and velocities taken by the planet. Small, but as it turns out, measurable. In particular, the velocity is the thing–the velocity of the star turns out to be the thing that you can measure. And so that’s how you find–determine that there’s an exoplanet there. What you do is you look for the reflex motion of the star. Planets going around the star, stars going around the center of mass also. And that is a motion that now, these days, can be observed. And you can see why this might have happened only very recently, because that motion is really very small.

Let me give you some masses just to give you a sense of this. I’ve already written down that the Sun’s mass is about 2 x 1030 kilograms. Just for reference, the Earth’s mass is 6 x 1024 kg, so down by almost a factor of a million. And so the Sun moves much slower than the Earth does, due to their mutual gravity. Jupiter is the most massive of the planets, and it’s at about 2 x 1027 kg, so 1,000 times smaller than the Sun. And so, of course, the Sun moves 1,000 times less because of Jupiter than it does because of the Earth.

Now, of course, the Sun actually responds to all of these planets, so it’s actually executing some complicated motion, which is the sum of the motions induced by all the planets. But in fact, Jupiter is significantly more massive than the rest of the planets, so by far the dominant motion that the Sun goes through has to do with the orbit of Jupiter. And so the consequence of this, because the masses are so much smaller, is that the velocity of the star is much, much less. These two less-than signs [<<] means much, much less than the velocity of the planet. But it can nevertheless be detected.

Okay, now, what do we expect to see? Supposing you can now go out and through means that we’ll actually talk about on Thursday, actually measure the velocities of stars in response to planets. What do you expect to see in those–in other stars? And basically, the answer to that is, what you expect to see depends on what your expectations for Solar Systems are. We’ve got one example, or at least ten years ago, we had only one example. And so, you have to take what you know about our own Solar System and infer what other Solar Systems might want to look like. And so, at this point I want to show you some things about our own Solar System, so a little slide show of the Solar System here. All right, don’t take notes, I’ll tell you everything you need to know after we finish the pretty pictures.

Okay, so starting from the innermost part of the Solar System. This is the innermost planet, this is the planet Mercury. Looks much like the Moon: it’s basically a rock with craters on it. There was a time when we thought that its spin period was exactly the same as its orbital period, so it keeps one face to the Sun. There’s all kinds of science fiction based on that–that turns out not to be true. But basically, it’s kind of a hot rock, that’s all you need to know about Mercury. Let’s see, oh, here’s a close up of a little piece of Mercury surface, and you would have a tough time telling that this was Mercury rather than the Moon or many other objects–rocky objects in the Solar System.

Next one out is Venus. Venus looks quite different because it’s got a very thick atmosphere, very thick carbon dioxide atmosphere. This is all clouds that you’re looking at here. And, in fact, the greenhouse effect, which is supposed to be responsible, perhaps, for global warming, was first studied and identified on Venus, because it appears to have run amok on Venus. The surface of Venus is extremely hot and it’s covered with these really thick clouds. So, it was quite hard, for a long time, to get a handle on what was going on down on the surface. This has now however been accomplished. They’ve put things into orbit that have radar, and can view the topography through the clouds. They’ve also dropped things onto the surface of Venus. The problem is at 700 degrees, and it rains sulfur down there, so it’s an unpleasant environment for machinery. So things don’t last very long. But nevertheless, they’ve gotten some information. Here’s a little Venus landscape. It’s entirely artificially colored, right? But the topography comes from these radar mapping missions. Here is a map of the whole of Venus made by these orbiting missions. And so Venus is important primarily for its atmosphere, and as a kind of warning for what might potentially one day happen here, if we’re not careful.

Okay, this is the third rock from the Sun. Ninety nine percent of all Yale courses deal with what’s going on on this little piece of cosmic debris. I’m not going to say anymore about it, therefore, oh, except for one thing: it comes with this companion object. This is the Moon. The Moon is a very special thing because, relative to its planet, it’s huge. This really shouldn’t be thought of as a planet and a moon, but rather as a double planet. Here they are to scale, and that’s much closer in size than any other moon-planet system around the major planets.

Moving outwards, we come to Mars. This is about as good an image of Mars as you can get from the Earth, and you can see why people got excited about it. These blotchy things here turn out to change with time. And in fact, they change with the Martian seasons. So people got very excited, thought, “oh my goodness, it’s vegetation,” you know, the seasons come, go. And there’s a polar ice cap up there, obviously. And 100 years ago, people somehow convinced themselves that there were canals and maybe cities, and maybe people all over this planet. This turns out to be wrong. It isn’t vegetation. It’s actually dust storms–that changes what you see. And by now we have some much more close up views from things like the Viking Missions and a number of more recent missions. And this is basically what the surface of Mars looks like. It has this slight reddish tint overall, and it’s a bunch of rocks. It has an atmosphere, although it’s less thick than the Earth’s atmosphere.

Now, one of the interesting things about Mars is you can see features that look like this. And this looks very much like river deltas. You know, you see these little tributaries coming into a big river, this kind of looks like Louisiana, or something like that. And so, people are pretty much convinced that there was once running water on Mars. And that’s important, because it is thought that the existence of life as we know it is dependent on the existence of liquid water. For a long time, people thought that there was no liquid water. Now, on Mars–it turns out that the particular temperature and atmospheric pressure that exists on Mars means that water goes from the solid state, from ice and sublimes, directly into the gaseous state, much like carbon dioxide does here. That’s why it’s called dry ice: because carbon dioxide, when you freeze it, and then warm it up again, turns directly into gas. Water is supposed to do the same on Mars, but there was, just a month ago, this interesting picture published. This is from a satellite orbiting Mars that’s been taking a lot of pictures. This is pictures of two identical parts of the Martian surface, one from 1999, one from 2005. And the claim is that there’s new stuff down here, and that the way and the pattern of that new stuff, and the way it must have come on, is from stuff flowing downhill, down the side of this crater. And so now, people are thinking, maybe there is something flowing around on Mars, although clearly not all that much of it. But that would be exciting if it was confirmed.

Okay, out beyond Mars is the asteroid belt, filled with rocky chunks of stuff that look vaguely like this–many, many of them. There are asteroids all over the Solar System. Most of them are between the orbits of Mars and Jupiter, but there are other families that are elsewhere. Some of these other families, it has been suggested, come from the asteroid belt, but they’ve had collisions or other catastrophes, and have been bumped into different orbits. But most of the asteroids are between Mars and Jupiter.

Now, out beyond the asteroid belt are a number of other planets, and much of what we know about these other planets come from a couple of satellites that look kind of like this. These are the Voyager satellites that were launched in the 1970s and have been traveling through the Outer Solar System ever since. This is a clever thing that they did. It turned out that in the ’70s and ’80s, the outer planets, Jupiter, Saturn, Uranus, and Neptune were aligned in such a way that one satellite could catch them all as they went past. And each time it goes past one of these things, it uses the gravitational attraction of that planet to swing itself to the next one, and then to the next one, and then to the next one. And so, these wonderful satellites, for many years, gave us pictures of one planet after another, which we now know quite a lot more about than we used to.

So, here’s Jupiter, this is by far the most massive planet. Here you’ve got the famous red spot. All that you see here is atmosphere, and it’s got very elaborate weather. And the red spot, it turns out to be a hurricane that has persisted for about 350 years. To us, it seems like an almost permanent feature, although it’s gotten fainter recently. But it’s sort of as if–supposing you were a race of creatures whose lifetime was about half a day, and you were observing the Earth. And you observed it for many lifetimes, and you saw the same hurricane sitting down somewhere in the Caribbean. You would think that that little spot was kind of a permanent feature, and that seems to be what this is. It’s a sort of really long-lasting hurricane. If you have–they have time-lapse movies of this. You can find them on the Internet, where you can see that the wind is actually circulating there.

Jupiter has moons, many of them–these–the four big ones you see here are the so-called “Galilean” moons, because they were discovered by Galileo. They’ve also included a little one. There are many dozens of moons this size. The moons–each of the moons has its own peculiar characteristics. I’m quite fond of this one, this is the innermost moon. It’s called Io, sometimes referred to as the pepperoni pizza moon. And it’s got the most elaborate volcanoes anywhere in the Solar System. It spews up sulfur all the time. And then, this sulfur sort of melts and flows all over the surface, and that’s what gives it its particular color. Each of the other moons has interesting characteristics of its own. Here’s an interesting thing that the Voyager satellites discovered: they discovered that Jupiter has rings. It was not thought that Jupiter had rings; from the Earth, you can’t see them. But from close up, it became apparent that Jupiter has rings the same way Saturn does.

But, of course, the Saturn rings are the most spectacular. Here’s Saturn, the next planet out. You can see that it, too, has weather-banded things down here. And then it has these very spectacular rings, seen here from various different angles as Saturn goes through its orbit. And these rings, we know now, are made up of individual little chunks of things. The Voyager Mission, this is obviously artificially-colored so that you could see all the different rings. And each one of those rings is made up of many, many, many little rocks. Saturn too has moons. This is Titan, Saturn’s big moon. And you can see from here, in this particular picture, that Titan has an atmosphere. That makes it very interesting. People have the feeling that at places where there are atmospheres are potential sources of life, and so people find Titan an interesting moon. I actually like this one better. This is Mimas. It’s just a rocky moon, but you can see it’s kind of got the great grandmother of all craters up there. This thing got slammed into by some asteroid that was just barely not big enough to blow the whole thing apart, but it raised this big pucker on the side of the moon.

Moving out to the next planet, which is Uranus. In ordinary light, you can’t actually see any features. Again, you’re looking at the atmosphere; there are clouds. But this picture was taken in a particular kind of red light, which brings out the cloud features. And what you can see is, it’s got a banded structure, the same as Jupiter and Saturn, but interestingly, the bands are on its side. One of the peculiar features of Uranus is that it rotates sideways, rather than kind of up and down, uniquely among the planets. This planet, too, has rings. It also has moons. Here’s another favorite moon of mine, this is Miranda. And this looks like what happened was that it actually did get blown apart by some impact, but then fell back together again. And you can see that it looks like it’s been chopped into pieces and then sort of thrown back into–together again.

Moving out to the next planet, here’s Neptune. Neptune, again, has weather. Here is the big, dark spot on Neptune, similar to the big, red spot on Jupiter. This little cloud here is called Scooter, because it moves faster than the rest of the weather on Neptune. It’s actually kind of a mystery how come Neptune has all this weather, because it’s very cold out there. There isn’t a whole lot of energy that should be in the atmosphere, and nobody can quite figure out how this is supposed to work. Neptune has moons. Here’s Neptune’s biggest moon, this is Triton. You can see this sort of frontier here between two types of topography, sort of moves around, and that is thought to be due to weather of various kinds–methane snow, stuff like that. And then, by now, this was the last planet Voyager II examined, and then it went past and it took this–took a lovely shot looking back at the Solar System. Here is Neptune and Triton as the Voyager Mission moved out into the Outer Solar System beyond the large planets.

### Chapter 4. Kuiper Belt Objects and Oort Cloud [00:29:47]

Now, Pluto wasn’t aligned properly to take part in this. This is–these are pictures of Pluto. This is a picture of Pluto from the ground. Here it is from the space telescope. As you can see, it’s got a moon. This is kind of all we know about Pluto at the moment. There is a spacecraft that is currently on route to Pluto, and when it arrives there a couple of decades from now–a decade from now, I guess, we’ll know much more.

Now, you may be aware that there was a little bit of fuss about Pluto over the summer. Let me show you why. Here it is. This is what all the fuss is about. This is three pictures, sort of shown as a movie. See this thing over here that’s moving? That’s moving because, unlike all the other objects in this picture, that thing is not a distant star. That is a planet in the Outer Solar System. This is the one that for a while was called Xena, now it’s called Eris. And these are the discovery photographs of this. Then as they kept taking pictures, they could plot the orbit. They determined the orbit of the thing using an equation you’ve already discovered. They figure out how far away it is. From how far away it is and how bright it is, you can figure out how big it is. And the problem that Eris presents is, it’s bigger than Pluto. And there are a whole bunch of other things out there that have also been discovered, also as big, or bigger than Pluto. Here are the eight currently largest-known, so-called “trans-Neptunian” objects, sometimes also called Kuiper Belt objects. Here’s Eris, here’s Pluto, here are a bunch of other ones. They find these things, like, one or two of them a year, now, and so we can confidently expect that twenty years from now, there are going to 100 such things. So you have the problem–oh you should–it’s worth nothing, these things also have moons, and some of them have a moon-to-planet ratio similar to the Earth and the moon. But here’s the Earth for scale. All of them, including Pluto and Eris, very much smaller than the Earth.

And so you’ve got yourself a problem. If you’re going to count Pluto, how do you not count these other things? And so the first suggestion was, well, you ought to count Pluto, and you ought to count a bunch of these other things too. And so, there was going to be twelve planets, or something like that, and presumably, many more to come. And then people decided, you know, the heck with this whole bunch of things. We’ll give them a different name; we won’t call them planets at all. And that was what was finally determined, leading to all sorts of, you know, “Save Pluto” campaigns from disgruntled third-graders who–[laughter] yes, thank you very much–who were unhappy about having to re-memorize all the mnemonics that tell you the planets in the Solar System. Actually, if you included things like Quaoar, you’ll get yourself into trouble trying to learn them all. And they’re finding lots of lots of these things.

Okay, so just a little about the geography of the Solar System. Here’s the Inner Solar System out to the orbit of Jupiter, with the asteroids and stuff. Here’s the Outer Solar System, starting with Jupiter. Here’s Pluto, and you’ll notice that Pluto really is an elliptical orbit, much more elliptical than the other planets. It also turns out the orbit is tipped. All the other planets–the orbits are in the same plane as each other. Pluto’s orbit is tipped out of the plane. Here’s one of the–these other trans-Neptunian objects. Here’s its orbit, and you can see that it goes way, way out and is highly elliptical, very much out of the plane. This is, in general, true of these, what are now called “dwarf planets,” or “trans-Neptunian objects,” or “Kuiper Belt objects.” But it’s also true that there is a further component of the Solar System even beyond that. Here’s Sedna’s orbit–that again is Sedna’s orbit, so we’ve now gone up a degree in scale. And then there’s this whole cloud of stuff out there called the Oort cloud and this is–this is where the comets live.

Comets are sort of ice balls. We can’t see them individually in the Oort cloud, but sometimes they collide with each other, and one of them falls into the Inner Solar System. And as it does, it heats up. It’s made out of ice, the ice melts, gets–streams backward, and we get these very spectacular things. Comets cause all kinds of mayhem, they kill dinosaurs, for example, and recently we’ve seen an example of this. This is a comet called Shoemaker-Levy that’s broken up into a bunch of pieces. And in 1994, those pieces slammed into Jupiter, and here you see the various–the effect of the various pieces of this comet that hit Jupiter. And so, comets can be dangerous things when they land on your planet.

Okay, this is the Outer Solar System again. Here’s the tipped orbit of Pluto. That’s the orbit of Neptune. Here’s Voyager I, Voyager II traveling now outward. They are escaping from the Solar System. And the very edges of the Sun’s influence, where you might say the Solar System ends and the interstellar medium begins, are indicated here. Each of these things means a slightly different boundary to the Sun’s influence. But the Voyagers are going to get there sometime in our lifetime, so they will be the first man-made things that actually pass out of our Solar System. And that’s as far as this goes. Let me turn the lights back on, here.

### Chapter 5. Classification and Interpretation of Celestial Objects [00:35:13]

Okay, so much for the Solar System. Now. Here’s the important question: I just threw a whole bunch of facts at you. Facts, information, pretty pictures. We have a lot of facts, and information, and pretty pictures. The Voyagers took many thousands of pictures and other–acquired other forms of information, as well. There have been many other spacecraft flying around our Solar System for quite some time. We know a lot of things. Here’s the question: now that you know all this stuff, what do you do with that information? So, what do you do with all this Solar System information? One thing you could do is, write it all down and memorize it. I strongly advise against that. You know, this is what the Internet is for. There are a whole bunch of websites out there that’ll tell you everything you could possibly want to know about all of the objects and all of the planets, moons, everything else. So that isn’t actually a productive way, I don’t think, of spending your time. So, what would you rather do? What would one–what would be a more productive way of thinking about this material?

And now, I want to step back and remind you of how science works. Remember the scientific method? They probably taught you something about this when you were, like, eleven or twelve years old. And you’ll remember how this works. So, scientific method–so, you have a hypothesis, which is a fancy word for a guess. Maybe you have competing hypotheses. And on the basis of this hypothesis you formulate some kind of experiment. Good experiments are sometimes called “controlled” experiments. And on the basis of the results of the experiment, you determine how much you believe the hypothesis, or which hypothesis you believe. And then, you know, you may modify your hypothesis or change it all together, and then do more experiments. And by iterating this procedure, you develop an understanding of whatever it is you’re trying to think about. Okay.

No. Not really, certainly not in astronomy. Astronomy doesn’t work this way at all. Think about what an experiment would be in astronomy. Okay, so here’s a ball of gas the size of the Sun made out of pure hydrogen. Here’s another ball of gas the size of the Sun made out of pure helium. We stick them in the sky and watch them evolve for ten billion years, okay. No, you can’t do experiments in astronomy. It doesn’t work that way at all. So, there has to be some other way of approaching it. And the reason for this is, is that astronomy is not an experimental science, it’s an observational science. This is true of many other sciences. A lot of biology, particularly environmental, or ecological aspects of biology, work this way. All of the social sciences. You can’t do controlled experiments in child development, that’s just ugly. And so, these are observational sciences, and this has a different methodology. And it starts, as its name would suggest, with observations. And you go out and you find a bunch of things, whether they’re butterflies, or planets, or whatever they are. And what do you do when you’ve found a whole bunch of things? What’s the next step?

The next step is classification. You put them into categories. And it’s important to get this right. If you’re dealing with, for example, animals, and you classify them as things that live in the ocean and things that live on land, then you’re going to have fish and whales in the same category. And you’re going to have lizards and frogs and snakes in the same category with people and bears, and things like that. And this isn’t going to lead you to a deep understanding, because you haven’t got the categories right. And so the classification is very important. And what it leads to when you get it right is a useful interpretation of what is going on. This is–interpretation, it’s just a fancy word for a good story. These are the kinds of things we write down in textbooks. The astronomers sometimes dignify this with the fancy word “scenario.” But it’s basically a story. And on the basis of this story, you say, well, we better check out, let’s see whether this story holds up. Let’s do more observations, and–all of these connections work in all directions. And so, this is actually a better description of how an observational science is done than this kind of thing up here.

So now, having gone through that little piece of philosophy, let’s apply it in practice. I’ve just described to you some observations, some observations of the Solar System. Now, what I’m going to do is, I’m going to take those objects that I just showed you and classify them, or attempt to. So let us classify–these are now going to be categories of Solar System objects. So, I claim, or at least, it’s the start of a good story, that there are six categories of objects in the Solar System.

The first is the Sun, which I didn’t actually show you, which is 99% of all of the mass in the Solar System, and almost all of the energy and heat, and so forth.

Then there are the inner planets, sometimes called the terrestrial planets, because the Earth is the prime example. And these are rocks, basically that would be silicon, iron, elements like that, with a very thin surface coating of ice, in some cases melted, in some cases gaseous. What I mean by ice is not necessarily water-ice, but also things like ammonia, and methane, and other compounds of that kind. This coating is very thin. If you had a scale model of the Earth that was six feet across and you put your hand up against it, you wouldn’t even feel that it was wet from the oceans. So, not very much. And these have masses, these things, these inner planets of, I don’t know, 10-7 to 10-5 of the Sun. And they’re in basically circular orbits.

Then you’ve got the asteroids, which are irregular, very small rocks in–mostly in orbits between Mars and Jupiter. And out beyond the asteroids, you’ve got the outer planets, sometimes called the “Jovian” planets, because Jupiter is the prime example of these things. And these are quite different from the inner planets. They’ve got a lot of gas and ice, both gas, which I’m defining here to be hydrogen and helium, and ice, which I’m defining to be, as I said, water, ammonia, methane, and similar compounds. And these are more massive, 10-4 to 10-3 of the Sun. They have rings, many moons, also circular orbits. And all these orbits, both these planets and these planets are basically in one plane.

Out beyond the Jovian planets are these trans-Neptunian objects, or Kuiper Belt objects. This is Pluto, etcetera. We don’t really know what they are yet, but it seems like they’re probably going to be rocky, just to judge from their size and mass. So we think they’re rocky. They’re a low mass, less than 10-7 of the mass of the Sun. And they are in elliptical and inclined orbits, way out there.

And then, finally, in the outer region, in this so-called Oort cloud, you have the comets, which are little snowballs. Balls of ice, again, ice in this generic sense.

So, here are the six categories that I would claim exist in the Solar System. And here’s my problem with the whole Pluto debate. The Pluto debate was basically about whether these guys are going to count as planets. But the thing is, “planets” is already a bad description, because it contains two quite different categories; namely, these inner terrestrial planets, and the outer Jovian planets. So, it seems to me that arguing whether category five should be part of some category that already contains two fundamentally different kinds of objects is kind of a strange argument to be having. Either we should split these two things off from each other, or, if we’re going to join these two kinds of the categories, fine, bring in anything you like. I don’t care, add the asteroids, too. And, in fact, in the original proposal, one of the asteroids qualified as well. And so, it doesn’t seem to me that this controversy was really paying justice to an appropriate classification of the things in the Solar System.

Okay, so now. Having classified it, the next step is interpretation. And, I think I will leave that–let’s see, how are we doing for time? Yes, I’ll leave that one for next time. So the question is going to be: now that you have these six kinds of objects, what is the story you tell about how the Solar System evolved? And what, in turn, does that tell you about what other kinds of planets you should see around other stars?

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